Covalent attachment of hydrophilic polymers, such as polyalkylene glycol polymers, also known as polyalkylene oxides, to biologically-active molecules and surfaces is of interest in biotechnology and medicine.
In particular, much research has focused on the use of poly(ethylene glycol) (PEG), also known as or poly(ethylene oxide) (PEO), conjugates to enhance solubility and stability and to prolong the blood circulation half-life of molecules.
In its most common form, PEG is a linear polymer terminated at each end with hydroxyl groups:HO—CH2CH2O—(CH2CH2O)n—CH2CH2—OH.
The above polymer, alpha-, omega-dihydroxylpoly(ethylene glycol), can also be represented as HO-PEG-OH, where it is understood that the -PEG-symbol represents the following structural unit:—CH2CH2O—(CH2CH2O)n—CH2CH2—where n typically ranges from about 4 to about 10,000. PEG is commonly used as methoxy-PEG-OH, or mPEG, in which one terminus is the relatively inert methoxy group, while the other terminus is a hydroxyl group that is subject to ready chemical modification. Additionally, random or block copolymers of different alkylene oxides (e.g., ethylene oxide and propylene oxide) that are closely related to PEG in their chemistry can be substituted for PEG in many of its applications.
To couple PEG to a molecule of interest, it is often necessary to activate the PEG by preparing a derivative of the PEG having a reactive functional group at least at one terminus. The functional group is chosen based on the type of available reactive group on the molecule that will be coupled to the PEG.
PEG is a polymer having the properties of solubility in water and in many organic solvents, lack of toxicity, and lack of immunogenicity. One use of PEG is to covalently attach the polymer to insoluble molecules to make the resulting PEG-molecule “conjugate” soluble. For example, it has been shown that the water-insoluble drug paclitaxel, when coupled to PEG, becomes water-soluble. Greenwald, et al., J. Org. Chem., 60:331-336 (1995).
The prodrug approach, in which drugs are released by degradation of more complex molecules (prodrugs) under physiological conditions, is a powerful component of drug delivery. Prodrugs can, for example, be formed by bonding PEG to drugs via linkages which are degradable under physiological conditions. The lifetime of PEG prodrugs in vivo depends upon the type of functional group(s) forming linkages between PEG and the drug. In general, ester linkages, formed by reaction of PEG carboxylic acids or activated PEG carboxylic acids with alcohol groups on the drug hydrolyze under physiological conditions to release the drug, while amide and carbamate linkages, formed from amine groups on the drug, are stable and do not hydrolyze to release the free drug. It has been shown that hydrolytic delivery of drugs from PEG esters can be favorably controlled to a certain extent by controlling the number of linking methylene groups in a spacer between the terminal PEG oxygen and the carbonyl group of the attached carboxylic acid or carboxylic acid derivative. For example, Harris et al., in U.S. Pat. No. 5,672,662, describe PEG butanoic acid and PEG propanoic acid, and activated derivatives thereof, as alternatives to carboxymethyl PEG for compounds where less hydrolytic reactivity in the corresponding ester derivatives is desirable. See, generally, PCT publication WO 01/46291.
One factor limiting the usefulness of proteinaceous substances for medical treatment applications is that, when given parenterally, they are eliminated from the body within a short time. This elimination can occur as a result of degradation by proteases or by clearance using normal pathways for protein elimination such as by filtration in the kidneys. Oral administration of these substances is even more problematic because, in addition to proteolysis in the stomach, the high acidity of the stomach destroys these substances before they reach their intended target tissue. The problems associated with these routes of administration of proteins are well known in the pharmaceutical industry, and various strategies are being employed in attempts to solve them. A great deal of work dealing with protein stabilization has been published. Various ways of conjugating proteins with polymeric materials are known, including use of dextrans, polyvinyl pyrrolidones, glycopeptides, polyethylene glycol, and polyamino acids. The resulting conjugated polypeptides are reported to retain their biological activities and solubility in water for parenteral applications.
Of particular interest is increasing the biological activity of interferons while reducing the toxicity involved with use of these proteins for treating human patients. Interferons are a family of naturally-occurring small proteins and glycoproteins produced and secreted by most nucleated cells in response to viral infection as well as to other antigenic stimuli. Interferons render cells resistant to viral infection and exhibit a wide variety of actions on cells. They exert their cellular activities by binding to specific membrane receptors on the cell surface. Once bound to the cell membrane, interferons initiate a complex sequence of intracellular events. In vitro studies have demonstrated that these include the induction of certain enzymes; suppression of cell proliferation, immunomodulation activities such as enhancement of the phagocytic activity of macrophages; augmentation of the specific cytotoxicity of lymphocytes for target cells; and inhibition of virus replication in virus-infected cells.
Interferons have been tested in the treatment of a variety of clinical disease states. The use of human interferon beta has been established in the treatment of multiple sclerosis. Two forms of recombinant interferon beta, have recently been licensed in Europe and the U.S. for treatment of this disease: interferon-beta-1a (AVONEX®, Biogen, Inc., Cambridge, Mass. and REBIF® Serono, Geneva, Switzerland) and interferon-beta-1b (BETASERON®, Berlex, Richmond, Calif.). Interferon beta-1a is produced in mammalian cells using the natural human gene sequence and is glycosylated, whereas interferon beta-1b is produced in E. coli bacteria using a modified human gene sequence that contains a genetically engineered cysteine-to-serine substitution at amino acid position 17 and is non-glycosylated.
Non-immune interferons, which include both alpha and beta interferons, are known to suppress human immunodeficiency virus (HIV) in both acutely and chronically-infected cells. See Poli and Fauci, 1992, AIDS Research and Human Retroviruses 8(2):191-197. Due to their antiviral activity, interferons, in particular alpha interferons, have received considerable attention as therapeutic agents in the treatment of hepatitis C virus (HCV)-related disease. See Hoofnagle et al., in: Viral Hepatitis 1981 International Symposium, 1982, Philadelphia, Franklin Institute Press; Hoofnagle et al., 1986, New Eng. J. Med. 315:1575-1578; Thomson, 1987, Lancet 1:539-541 Kiyosawa et al., 1983, in: Zuckerman, ed., Viral Hepatitis and Liver Disease, Allen K. Liss, New York pp. 895-897; Hoofnagle et al., 1985, Sem. Liv. Dis., 1985, 9:259-263.
Interferon-polymer conjugates are described in, for example, U.S. Pat. No. 4,766,106, U.S. Pat. No. 4,917,888, European Patent Application No. 0 236 987, European Patent Application No. 0 510 356 and International Application Publication No. WO 95/13090.
Chronic hepatitis C is an insidious and slowly progressive disease having a significant impact on the quality of life. Despite improvement in the quality of the blood-donor pool and the recent implementation of testing of donated blood for HCV, the estimated incidence of acute infection among persons receiving transfusions is 5 to 10%. See Alter et al., in: Zuckerman, ed., Viral Hepatitis and Liver Disease, Allen K. Liss, New York. 1988, pp. 537-542. Thus, of the approximately 3 million persons who receive transfusions in the United States each year, acute hepatitis C will develop in about 150,000. While many patients who contract hepatitis C will have subclinical or mild disease, approximately 50% will progress to a chronic disease state characterized by fluctuating serum transaminase abnormalities and inflammatory lesions on liver biopsy. It is estimated that cirrhosis will develop in up to about 20% of this group. See Koretz et al., 1985, Gastroenterology 88:1251-1254.
Interferons are known to affect a variety of cellular functions, including DNA replication, and RNA and protein synthesis, in both normal and abnormal cells. Thus, cytotoxic effects of interferon are not restricted to tumor or virus-infected cells but are also manifested in normal, healthy cells. As a result, undesirable side effects may arise during interferon therapy, particularly when high doses are required. Administration of interferon can lead to myelosuppression, thereby resulting in reduced red blood cell count, and reduced white blood cell and platelet levels. Interferons commonly give rise to flu-like symptoms (e.g., fever, fatigue, headaches and chills), gastrointestinal disorders (e.g., anorexia, nausea and diarrhea), dizziness and coughing. Often, the sustained response of HCV patients to non-PEGylated interferon treatment is low and the treatment can induce severe side effects, including, but not limited to, retinopathy, thyroiditis, acute pancreatitis, and depression.
The undesirable side effects that accompany interferon therapy frequently limit the therapeutic usefulness of interferon treatment regimes. Thus, a need exists to maintain or improve the therapeutic benefits of such therapy while reducing or eliminating the undesirable side effects.